The present invention relates generally to a method for activating zeolite catalysts and, more particularly, to a method for removing organic cations from organic cation-containing crystalline zeolites. The present invention further relates to the conversion of by-products resulting from the removal of the organic cations to environmentally safe products.
Zeolite materials, both natural and synthetic, contain catalytic properties which make them extremely useful in various types of hydrocarbon conversions. These zeolite materials have been used as catalysts under various operating conditions to effect such reactions as cracking, hydrocracking, dewaxing, reforming, hydroforming, isomerization, polymerization, hydroisomerization and alkylation, just to name a few.
Some of the synthetic zeolites which are useful in the conversion of hydrocarbons, especially, but not exclusively in those reactions mentioned above, contain as synthesized organic cations or a mixture of organic cations and metal cations.
When the synthetic zeolites are prepared in the presence of organic cations they are often initially catalytically inactive. It is believed that this catalytic inactivity can be ascribed to the presence in the intracrystalline free space of the zeolite of organic cations from the reaction mixture. While it is not absolutely essential to employ reaction mixtures which contain organic cations in the synthesis of some zeolites, it is believed that when organic cations are present in the reaction, a synthetic zeolite of desired crystalline structure is produced.
The need to remove organic cations from synthetic zeolites to render them catalytically active has been recognized. However, a variety of the techniques heretofore employed are subject to several disadvantages, since the organic cation-containing zeolites may have undesirably low adsorptive properties and become amorphous or lose their crystalline structure if subjected to very elevated temperatures, such as those which may be necessary to remove the organic material in the channels or expel any adsorbed material. Thus, because of diffusion limitations, high temperature for extended periods of time are often required to oxidize the organic material in the channel system and such rigorous treatment tends to disrupt the zeolite framework, adversely affecting its stability and adsorptive properties.
One approach for removing organic cations from organic cation-containing zeolites has been described in U.S. Pat. No. 4,187,283. The method described includes the sequential steps of (1) heating the organic cation-containing zeolite at a temperature within the approximate range of 150.degree. to 600.degree. C. for a period of time sufficient to decompose organic material contained in the zeolite without effecting coking thereof, (2) contacting the resulting product with an aqueous solution of a monovalent metal salt at a temperature between about 50.degree. and about 120.degree. C. for a period of time sufficient to expel at least a major proportion of the decomposition products of said organic material and (3) calcining the product so obtained at a temperature of at least about 500.degree. C. but below a temperature at which crystallinity of the zeolite is adversely affected.
In U.S. Pat. Nos. 4,559,314 and 4,522,929 the organic cation-containing zeolite is activated, that is, the organic cations are removed by heating them in an inert atmosphere at 1000.degree. F. for one hour, for example, followed by base exchange with ammonium salts followed by calcination at 1000.degree. F. in air.
For example, it is known to contact the organic cation-containing zeolite with nitrogen, prior to calcination, in order to remove the organic cations therefrom. However, nitrogen precalcination typically results in the production of undesirable by-products. The particular product resulting from the nitrogen precalcination of the synthetic zeolite depends upon the type of organic component used in the zeolite synthesis. For instance, in the synthesis of zeolite beta and ZSM-12, reaction mixtures containing tetraethylammonium (TEA) cation are most typically employed. The products which result from the nitrogen precalcination of these organic cation-containing zeolites are ethylene, triethylamine, C.sub.4 + olefins, ammonia and alkylbenzenes. The triethylamine and aromatic emissions resulting from the nitrogen precalcination of zeolite beta and ZSM-12 present a problem regarding environmental considerations because of the negative effect these emissions have on the environment.
It is therefore an object of the present invention to provide a method for activating zeolite catalysts containing organic cations using steam to substantially decompose the organic cations.
It is a further object of this invention to convert by-products resulting from the removal of organic cations from zeolites to environmentally safe products.